1. Field of the Invention
This invention relates to a method for producing high energy electroluminescent devices capable of emitting visible light at room temperature, and more particularly to a method using homogeneous chemical vapor deposition and in-situ doping to produce amorphous silicon: hydrogen thin films having optical bandgaps between 1.6 and 2.6 eV.
2. Description of the Prior Art
Light emitting diodes are very important in numerous commercial applications, and are used in many types of displays. Typically, these devices are comprised of doped and undoped semiconductor layers across which an electric field is provided in order to inject carriers into the device. When this occurs, light is spontaneously produced in accordance with the bandgap of the layers which are chosen. As an example, different compositions of gallium-aluminum-arsenic and gallium-arsenide-phosphide are most often used to fabricate electroluminescent devices. Other III-V and II-VI semiconductor compounds are also used for this purpose.
It is generally important that the visible light be produced at room temperatures, so that the devices can be used in many consumer electronics products. In order to change the color (wavelength) of the light emitted by these diodes, it is generally necessary to change the material comprising the various layers of the devices. For example, one composition (x) of the alloy GaAs.sub.1-x P.sub.x will yield one color output, while another alloy composition can be used to yield another color. In these devices, it is not possible to use the same binary or ternary combination of elements to provide wavelengths which can be tunable over wide ranges.
Another problem with these well known electroluminescent diodes is that they are generally comprised of ternary compositions that are not easily integrated with other types of semiconductor circuitry, that are usually comprised of silicon or its alloys. This means that the devices are not readily combined with one another on the same wafer and that the processing conditions, such as temperature and deposition techniques, are not always compatible.
In order to provide additional classes of light emitting diodes which have advantages over these known systems, researchers have developed techniques for producing amorphous hydrogenated silicon (a-Si:H) films which will exhibit visible luminescence at room temperature. Thin films of this type are described in the following references: IBM Technical Disclosure Bulletin, Vol. 25, No. 3B, p. 1664, August, 1982, IBM Technical Disclosure Bulletin, Vol. 24, No. 3, p. 1523, August, 1981, D. J. Wolford et al, Physica 117B and 118B, pp. 920-922 (1983), D. J. Wolford et al, Appl. Phys. Lett., Vol. 42, No. 4, p. 369, Feb. 15, 1983.
In order to make suitable a-Si:H films, it is necessary to incorporate large amounts of hydrogen in the films in order to provide defect passivation in the amorphous silicon network. The presence of hydrogen results in a reduction of the density of states in the amorphous silicon energy gap as pointed out by S. C. Gau et al, Appl. Phys. Lett. 39 (5), p. 436, Sep. 1, 1981. Gau et al describes the deposition of device quality a-Si:H films by several techniques including the glow discharge decomposition of silicon bearing gas species (SiH.sub.4, Si.sub.2 H.sub.6, and SiF.sub.4). Other techniques for preparing a-Si:H films include sputtering of silicon in a hydrogen atmosphere (W. Paul et al, Solid State Communications, 20, 969 (1976)) and ion beam deposition (F. H. Cocks et al, Appl. Phys. Lett. 36, 909 (1980)).
Amorphous Si:H may also be produced by the pyrolytic decomposition of SiH.sub.4. Although this technique is well known, and is generally termed chemical vapor deposition (CVD), little has been published regarding the in-situ preparation of high quality (low spin density, photoconducting, hydrogen passivated) a-SiH films. Even though CVD is an exceptionally clean and well controlled process that is commonly used to prepare high quality polycrystalline and epitaxial silicon thin films, it has been relatively unsuccessful when used to prepare a-Si:H. One of the reasons for this is that high temperatures are required to pyrolyze SiH.sub.4. Generally, temperatures in excess of 500.degree. C. are required which cause hydrogen to be evolved from the growing a-Si:H film. In a CVD apparatus, the source gasses and the substrate are generally hot and are at about the same temperature. Thus, the hydrogen required for defect passivation in the amorphous silicon network is evolved from the growing film, which results in an amorphous silicon film displaying a high defect density. The Fermi level is thus pinned near mid-gap, precluding doping, and rendering the material unsuitable for most applications. Consequently, such CVD prepared films must be post-hydrogenated, which reduces the defect density to a level where efficient doping may occur.
One method for enhancing hydrogen incorporation during growth of thin films using CVD is to reduce the CVD operating temperatures. A technique for doing this is to use more easily pyrolyzed silicon sources, such as higher order silanes. Gau et al, referenced above, uses this approach. In Gau et al, a hot wall/hot substrate CVD technique was used in which amorphous silicon: hydrogen films were grown at temperatures as low as 375.degree. C. In their system, argon was used as a carrier gas and the films were doped in-situ by incorporating appropriate amounts of PH.sub.3 or B.sub.2 H.sub.6 in order to dope the films either n-or-p type, respectively.
In the technique of Gau et al, high temperature in-situ doping is used, but it is not possible to incorporate sufficient amounts of hydrogen in order to obtain optical bandgaps inthe visible range, i.e., between 1.6 and 2.6 eV. Such a bandgap is required to have visible light emission at room temperatures, but it cannot be achieved with the technique of Gau et al (for example, see FIG. 2 therein).
Another approach to providing a-Si:H films having a sufficient amount of hydrogen is a plasma discharge method. In this method, infrared photo and electroluminescence has been demonstrated with emission in the range 0.9-1.4 eV. These measurements have been made at liquid nitrogen temperatures, as can be seen by reference to J. I. Pankove et al, Appl. Phys. Lett., 29, 620 (November 1976) and R. A. Street, Advances in Physics, 30, 593 (1981). As a consequence of a high density of non-radiative recombination centers in plasma-prepared films, a small activation energy for thermal quenching of luminescence processes, and the low energies at which the luminescence occurs, the development of electroluminescent devices operating in the visible spectrum at room temperature was not feasible using plasma techniques.
Films of a-Si:H prepared by CVD are generally characterized by extreme insulating properties, having resistivities greater than 10.sup.11 .OMEGA.-cm. This requires that they be doped in order to be useful in device structures such as p-n junction devices, p-i-n structures, and Schottky barrier devices. While this can be done in-situ at high temperatures, the resulting films do not exhibit visible electroluminescence.
On the other hand, doping of amorphous materials at low temperatures has been unsuccessful in the past because of the high defect densities that would result. If the defect density of a material is too high, the dopants will not be able to give up their electrons or holes to the conduction or valence bands of the material, and the dopant will be rendered ineffective. In turn, this will yield a material with poor electronic properties.
Consequently, the present invention has as its primary object the development of a process for producing doped a-Si:H films which exhibit visible electroluminescence at room temperatures.
It is another object of the present invention to provide a method for producing electroluminescent devices incorporating at least one layer of doped a-Si:H which can be produced at substrate temperatures less than about 200.degree. C.
It is another object of the present invention to provide a technique for the low temperature preparation of a-Si:H films having sufficient amounts of hydrogen therein to provide a bandgap suitable for producing visible electroluminescence at room temperatures, where the a-Si:H layer can be doped in-situ to a desired conductivity.
It is another object of this invention to provide a low temperature process for producing a-Si:H films which can be doped to any desired conductivity level and type during the low temperature process in which the a-Si:H film is being deposited upon the substrate.
It is another object of the present invention to provide an improved process for producing a-Si:H films, where the films are characterized by high conductivities and electroluminescence in the range 1.6-2.6 eV.